Embodiments of the present invention relate to a method for manufacturing a highly-integrated semiconductor device, and more particularly to a method for manufacturing a semiconductor device having a buried word line that is stably operated in a highly-integrated semiconductor memory device.
A semiconductor memory device includes a plurality of unit cells each having a capacitor and a transistor. The capacitor is used to temporarily store data, and the transistor is used to transfer data between a bit line and the capacitor in response to a control signal (word line). The data transfer occurs using semiconductor properties where electrical conductivity changes depending on environments. The transistor has three regions, i.e., a gate, a source, and a drain. Electrical charges are moved between the source and the drain according to a control signal inputted to the gate of the transistor. The movement of the electric charges between the source and the drain occurs through a channel region.
A channel region of the transistor is formed in an active region of a semiconductor substrate, and the active region is defined by a device isolation film. The device isolation film may be formed by a shallow trench isolation (STI) process. In more detail, the STI process includes: forming a trench in the semiconductor substrate; and filling in the trench with insulation material.
As data storage capacity of the semiconductor memory device and the integration degree thereof are increased, the necessity of manufacturing a smaller-sized unit cell is increasing. The reduction in design rules requires a distance between one active region and a neighboring active region to be reduced. If the active region is reduced in size, a contact area coupling an overlying device element to the active region is reduced, and contact resistance between the active region and the contact is increased accordingly. In order to overcome the shortcomings and increase the integration degree, it is necessary to reduce a distance between the active regions without reducing the size of the active regions.
To make the distance between neighboring active regions shorter, the width of a device isolation trench defining each active region becomes narrower. Meanwhile, in order to reduce interference between the neighboring active regions, the trench must be deeply formed. Preferably, the depth of the device isolation trench is greater than the width of the trench. In order to prevent a void from being generated in a device isolation pattern, an insulation film should completely fill the device isolation trench in order to prevent an unnecessary electric field and to ensure stable operation of the semiconductor device.
Typically, before an insulation material fills up a trench in which a device isolation film for defining the active region is to be formed, an oxide film and a nitride film are formed in the trench. The oxide film and the nitride film are formed to prevent a Hot Electron Induced Punch-Through (HEIP) phenomenon in a PMOS transistor as well as to reduce a leakage current of a semiconductor device. The oxide film and the nitride film cause the trench width to be narrower, so that it is difficult to fill the trench with an insulation material without generating any voids. In addition, an oxide film formed in the trench may be formed by oxidation of the exposed semiconductor substrate, so over sidewalls of the trench are oxidized and the active region is reduced in size. However, if the oxide film is thinly formed, the purpose of preventing the HEIP phenomenon and leakage current cannot be properly achieved, resulting in deterioration of stability of the semiconductor device.